When working on intricate files, I use Save, Save_As and Save_As_a_Copy often. They have similarities, but also important differences.
Then, when I am doing any destructive editing, such as Image Size, etc., I always, always work from one of the Save_As, or Save_As_a_Copy files, and never my final, Layered PSD files.
A good Saving scheme can, well "save" you, down the line.
I have had the same thing happen, and usually when I hit Save, instead of Save_As (renaming incrementally), but with my other Saves, it's just a matter of muttering to myself, Closing the file, and Opening one of those various "backup files." For some reason, there have been times, where my index finger was not firmly on the Shift key, as it should have been - "oh shucks" time.
Good luck, and while there is not a fix for what happened, let it be a learning experience, and spend a moment with those different versions of Save, then introduce them into your workflow.